The present invention relates to a solar energy powered water heater and storage device. More particularly, the invention provides a low-profile, flat-plate solar energy collector holding a fluid which is heated and circulated to an integrated, insulated hot fluid storage tank, the hot fluid in turn heats line pressure water passing through an extended length, heat-exchange piping circuit incorporated within said tank, in conjunction with a supplementary, electric water-heating means.
A commonly-used device for electrically heating a continuous, moderate flow of water for domestic purposes, typically limited to 1.5 liter/minute, is fitted with a 3-5 kw heating element. Sucn devices are marketed in Israel under the name "ATMOR". A heating element of such capacity places a heavy strain on the electrical wiring of a typical residence, since a typical shower requires a flow of about 5 liter/minute, such devices are not satisfactory for supplying hot water for such needs.
Solar water heaters are also fitted with electric heater elements but, in contrast to the "ATMOR" type device, the water rarely requires electrically powered heating from ambient temperature, as even in the winter months some degree of water heating is obtained from solar radiation. Consequently, solar water heaters are fitted with a 2-2.5 kw heater element which, if operated for a sufficient length of time before hot water is required, is enough for all but extreme conditions. The electric heater is however situated far below the outlet providing hot water for consumption. Switching on the heater element when water is demanded, or a short time beforehand, will not provide hot water when needed and so will waste electricity.
The most widely used solar collectors and storage tanks operate by thermosyphon circulation and have a cylindrical storage tank positioned higher than the collector. The system operates wholly at line pressure and the stored liquid is water provided for consumption. An electric heater element positioned at the bottom of the tank boosts water temperature if so required.
The disadvantages of this system are known. Such devices are obtrusive architecturally, take up considerable roof space, need thick thermal insulation to reduce heat losses, and the storage tank and absorber are constructed of thick steel to resist line pressure which may be as high as 10 atmospheres in some localities. Several hours of electrical heating are required to provide hot water when insufficient solar radiation is available. The heater element continues to operate until the lower part of the storage tank holding a thermostat reaches the set-point temperature. The upper part of the tank is thus heated to well above to the temperature required by the user, and much electricity is wasted.
The architectural disadvantages and thermal insulation problems are overcome by solar water heaters having an integrated water storage tank. Integrated devices are described in U.S. Pat. No. 4,003,367 to Wikholm, U.S. Pat. No. 4,421,100 to Yu, U.S. Pat. No. 4,637,375 to Larkin, U.S. Pat. No. 5,462,047 to Kleinwachter, and in Israel Patents nos. 48 555 to Koor Metals and 53 396 to Shell Oil.
The Larkin device and the Shell Oil device allow line pressure to enter the storage tank, which must therefore be built sufficiently strong and of a shape able to resist such pressure. The Yu device is complicated by having 3 separate fluid circuits. The Koor Metals device has a complex collector and relies on radiation sensors to govern liquid circulation routes. The Wikholm device requires that substantially the whole storage volume be heated before hot water is obtained in the line pressure tubing. The Kleinwachter device provides only very low pressure supply to the consumer.
Systems having an integrated water tank and separate fluid circuits, the first such circuit including the absorber panel and the second circuit providing hot water for consumption, obtain certain advantages. One of these is the ability to use a fluid other than municipal water in the absorber panel circuit. Among the useful options for such fluid are:
a. the addition to the water of an anti-freeze substance; PA1 b. the addition of a corrosion inhibitor; PA1 c. the use of demineralized water to reduce the formation of deposits; and PA1 d. the use of a fluid other than water having more desirable heat-transfer properties.
Obviously, none of the above are feasible when the fluid in the absorber circuit is also being supplied for consumption. The concept of using two separate fluid circuits has long been known and is the central feature of a water heater disclosed by Bailey in U.S. Pat. No. 1,242,511, whose device is of the type having a spaced-apart absorber and storage tank.
A further consideration in the design of water heaters, using an electric heating element to provide some or all of the water heating capacity, is the importance of protecting the heating element against burn out. This can occur as a result of a water supply interruption of which the consumer is unaware, the continuing consumption causing the heating element to run dry and bum out. It is to be noted that while the cost of the replacement element may not be of much concern, the cost of its installation by an electrician will be several times the cost of the element itself.
AU-B-488085 discloses a solar heating apparatus comprising a closed box plate collector mounted so that one end thereof is higher than the other end; a tank mounted adjacent said plate collector so that a substantial part of said tank is higher than said collector and said tank being filled with a fluid heat-transfer medium; a pair of conduits for cold fluid heat-transfer medium flow arranged one adjacent each side of said collector communicating the lower part of said tank with the lower part of said collector; a substantially centrally-located conduit for heated fluid heat-transfer medium communicating the higher part of said collector with said tank; and a grid or coil of heat-conducting tubing located within said tank and having, disposed externally of said tank, an inlet thereto and an outlet therefrom.
However, this arrangement still leaves scope for improvement to provide a solar water heater capable of more rapid electrical temperature boosting.